The current basic knowledge on the interrelation of diseases is very limited, partly because the scientific method maximizes internal validity, but excludes patients with co-morbidity. The elderly patient is usually excluded from studies included in clinical guidelines. The Health Care System should dispose of mechanisms to ensure the correct representation in studies of the population who will ultimately receive pharmacological treatment. We should study profoundly ways to optimize the use of drugs with synergic effects in clusters or in a combination of diseases.